In a cellular radio communication network represented by a mobile phone, a wide service area is formed by distributing and arranging a plurality of radio base stations. Each radio station forms a radio cell, which is an area within which communication with each radio station is possible. Usually, about one to six radio cell(s) are managed by one radio base station. In addition, it is possible to continue communication by handover by which adjacent radio cells have an overlap in a part of coverage, even when a radio terminal (User Equipment: UE) moves over the radio cells.
At the time of installing or operating radio base stations, the coverage of a radio cell is optimized so as to reduce the areas in which communication is not possible, and further improve communication quality of UE, in particular, throughput. In general, a drive test using dedicated measurement instruments is performed in the field. In the drive test, received power or interference condition of radio wave, an abnormal communication cut-off or a handover failure, and throughput are measured. Then, an insufficient area of received power (Weak coverage), or an area where strong interference is received (Pilot pollution) is specified to adjust a radio parameter for solving such problems. The radio parameter to be adjusted is, for example, an antenna tilt angle of a radio cell, an antenna azimuth angle, transmitted power and a handover parameter.
The coverage optimization for a radio cell based on the above-described drive test causes an increase of operating costs for the radio communication network due to manual measurement or manual tuning operation of a radio parameter. For this reason, there has been proposed a technique for optimizing the coverage of a radio cell autonomously, so as to reduce costs for optimizing the coverage of the radio cell.
As a technique for optimizing the coverage of a radio cell autonomously, there is CCO (Coverage and Capacity Optimization) in a SON (Self Organizing Network). The SON is a technique for performing parameter settings or optimization for the radio communication network autonomously, and has been standardized in 3GPP (3rd Generation Partnership Project) as a technique for reducing CAPEX (Captital Expenditure) or OPEX (Operational Expenditure) for radio communication infrastructures. The CON is one of usage modes of the SON, and a technique for optimizing the coverage of a radio cell autonomously by collecting the measurement results of radio quality (e.g. RSRP: Reference Signal Received Power, and RSRQ: Reference Signal Received Quality) from the radio terminal and, changing the radio parameter (e.g. transmitted power or tilt angle of antenna) of the radio cell.
In addition, as other technique, there is ICIC (Inter-Cell Interference Coordination). The ICIC is a technique for avoiding interference by reducing transmitted power or stopping signal transmission in coordination with between the radio base stations based on radio quality information reported from a radio terminal.
By the way, it is necessary to evaluate the network quality and decide the necessity for changing a radio parameter in each radio cell, so as to optimize the radio parameter of the radio cells autonomously. Further, when actually changing the radio parameter, it is necessary to evaluate whether or not unexpected quality deterioration occurs due to the change.
As an example of the method for evaluating a radio parameter of the radio cells, there is a technique in Non-Patent Literature 1. According to the technique in Non-Patent Literature 1, the overall quality index that coordinates a plurality of radio cells is generated, and the effect for changing a tilt angle is evaluated using the index. For example, in the environment of 19 base stations and 57 cells, the overall quality index (U) is generated by the following formula, and a tilt angle in each radio cell is changed to maximize the value. Here, in the following formula, P is a set of radio cells and |P| represents 57, which is a total number of the radio cells. Sm,avg represents average throughput of the radio cell m, and Sm,edge represents cell edge throughput of the radio cell m. Further, Wavg and Wedge are the parameters for defining which of the average throughput and the cell edge throughput is more important.
  U  =            1                      P                      ⁢                  ∑                  m          ∈          P                    ⁢              (                                            w              avg                        ⁢                          s                              m                ,                avg                                              +                                    w              edge                        ⁢                          s                              m                ,                edge                                                    )            